General practice seems to favor a smaller allowance for shrinkage fits than for forced fits, although in many shops the allowances are practically the same in each case, and for some classes of work, shrinkage allowances exceed those for forced fits. In any case, the shrinkage allowance varies to a great extent with the form and construction of the part which has to be shrunk into place. The thickness or amount of metal around the hole is the most important factor. The way in which the metal is distributed also has an influence on the results. Shrinkage allowances for locomotive driving wheel tires adopted by the American Railway Master Mechanics Association are as follows:
| Center diameter, inches | 38 | 44 | 50 | 56 | 62 | 66 |
| Allowance, inches | 0.040 | 0.047 | 0.053 | 0.060 | 0.066 | 0.070 |
Whether parts are to be assembled by forced or shrinkage fits depends upon conditions. For example, to press a driving wheel tire over its wheel center, without heating, would ordinarily be a rather awkward and difficult job. On the other hand, pins, etc., are easily and quickly forced into place with a hydraulic press and there is the additional advantage of knowing the exact pressure required in assembling, whereas there is more or less uncertainty connected with a shrinkage fit, unless the stresses are calculated. Tests to determine the difference in the quality of shrinkage and forced fits showed that the resistance of a shrinkage fit to slippage was, for an axial pull, 3.66 times greater than that of a forced fit, and in rotation or torsion, 3.2 times greater. In each comparative test, the dimensions and allowances were the same.
The most important point to consider when calculating shrinkage fits is the stress in the hub at the bore, which depends chiefly upon the shrinkage allowance. If the allowance is excessive, the elastic limit of the material will be exceeded and permanent set will occur, or, in extreme cases, the ultimate strength of the metal will be exceeded and the hub will burst.
CHAPTER IV
THREAD CUTTING IN THE LATHE
When threads are cut in the lathe a tool t is used (see [Fig. 2]), having a point corresponding to the shape of the thread, and the carriage is moved along the bed a certain distance for each revolution of the work (the distance depending on the number of threads to the inch being cut) by the lead-screw S which is rotated by gears a, b and c, which receive their motion from the spindle. As the amount that the carriage travels per revolution of the work, and, consequently, the number of threads per inch that is cut, depends on the size of the gears a and c (called change gears) the latter have to be changed for cutting different threads. The proper change gears to use for cutting a given number of threads to the inch is ordinarily determined by referring to a table or “index plate” I which shows what the size of gears a and c should be, or the number of teeth each should have, for cutting any given number of threads per inch.